Regulation of cell proliferation, differentiation, and migration is important for the formation and function of tissues. Regulatory proteins such as growth factors control these cellular processes and act as mediators in cell-cell signaling pathways. Growth factors are secreted proteins that bind to specific cell surface receptors on target cells. The bound receptors trigger intracellular signal transduction pathways which activate various downstream effectors that regulate gene expression, cell division, cell differentiation, cell motility, and other cellular processes. Some of the receptors involved in signal transduction by growth factors belong to the large superfamily of G-protein coupled receptors (GPCRs) which represent one of the largest receptor superfamilies known.
GPCRs are biologically important as their malfunction has been implicated in contributing to the onset of many diseases, which include, but are not limited to, Alzheimer's, Parkinson, diabetes, dwarfism, color blindness, retinal pigmentosa and asthma. Also, GPCRs have also been implicated in depression, schizophrenia, sleeplessness, hypertension, anxiety, stress, renal failure and in several cardiovascular, metabolic, neuro, oncology and immune disorders (F Horn, G Vriend, J. Mol. Med. 76: 464–468, 1998.). They have also been shown to play a role in HIV infection (Y Feng, C C Broder, P E Kennedy, E A Berger, Science 272:872–877, 1996).
GPCRs are integral membrane proteins characterized by the presence of seven hydrophobic transmembrane domains which together form a bundle of antiparallel alpha (a) helices. The 7 transmembrane regions are designated as TM1, TM2, TM3, TM4, TM5, TM6, and TM7. These proteins range in size from under 400 to over 1000 amino acids (Strosberg, A. D. (1991) Eur. J. Biochem. 196: 110; Coughlin, S. R. (1994) Curr. Opin. Cell Biol. 6: 191–197). The amino-terminus of a GPCR is extracellular, is of variable length, and is often glycosylated. The carboxy-terminus is cytoplasmic and generally phosphorylated. Extracellular loops of GPCRs alternate with intracellular loops and link the transmembrane domains. Cysteine disulfide bridges linking the second and third extracellular loops may interact with agonists and antagonists. The most conserved domains of GPCRs are the transmembrane domains and the first two cytoplasmic loops. The transmembrane domains account for structural and functional features of the receptor. In most G-protein coupled receptors, the bundle of a helices forms a ligand-binding pocket formed by several G-protein coupled receptor transmembrane domains.
The TM3 transmembrane domain has been implicated in signal transduction in a number of G-protein coupled receptors. Phosphorylation and lipidation (palmitylation or farnesylation) of cysteine residues can influence signal transduction of some G-protein coupled receptors. Most G-protein coupled receptors contain potential phosphorylation sites within the third cytoplasmic loop and/or the carboxy terminus. For several G-protein coupled receptors, such as the b adrenoreceptor, phosphorylation by protein kinase A and/or specific receptor kinases mediates receptor desensitization. In fact, phosphorylation of an activated G-protein coupled receptor is a common mechanism for desensitizing signaling to a G-protein.
The extracellular N-terminal segment, or one or more of the three hydrophilic extracellular loops, have been postulated to face inward and form polar ligand binding sites which may participate in ligand binding. Ligand binding activates the receptor by inducing a conformational change in intracellular portions of the receptor. In turn, the large, third intracellular loop of the activated receptor interacts with an intracellular heterotrimeric guanine nucleotide binding (G) protein complex which mediates further intracellular signaling activities, including the activation of second messengers such as cyclic AMP (cAMP), phospholipase C, inositol triphosphate, or ion channel proteins. TM3 has been implicated in several G-protein coupled receptors as having a ligand binding site, such as the TM3 aspartate residue. TM5 serines, a TM6 asparagine and TM6 or TM7 phenylalanines or tyrosines have also been implicated in ligand binding (See, e. g., Watson, S. and S. Arkinstall (1994) The G-protein Linked Receptor Facts Book, Academic Press, San Diego Calif., pp. 2–6; Bolander, F. F. (1994) Molecular Endocrinology, Academic Press, San Diego Calif., pp. 162–176; Baldwin, J. M. (1994) Curr. Opin. Cell Biol. 6: 180–190; F Horn, R Bywater, G Krause, W Kuipers, L Oliveira, ACM Paiva, C Sander, G Vriend, Receptors and Channels, 5:305–314, 1998).
Recently, the function of many GPCRs has been shown to be enhanced upon dimerization and/or oligomerization of the activated receptor. In addition, sequestration of the activated GPCR appears to be altered upon the formation of multimeric complexes (AbdAlla, S., et al., Nature, 407:94–98 (2000)).
Structural biology has provided significant insight into the function of the various conserved residues found amongst numerous GPCRs. For example, the tripeptide Asp(Glu)-Arg-Tyr motif is important in maintaining the inactive confirmation of G-protein coupled receptors. The residues within this motif participate in the formation of several hydrogen bonds with surrounding amino acid residues that are important for maintaining the inactive state (Kim, J. M., et al., Proc. Natl. Acad. Sci. USA, 94:14273–14278 (1997)). Another example relates to the conservation of two Leu (Leu76 and Leu79) residues found within helix II and two Leu residues (Leu 128 and Leu131) found within helix III of GPCRs. Mutation of the Leu128 results in a constitutively active receptor—emphasizing the importance of this residue in maintaining the ground state (Tao, Y. X., et al., Mol. Endocrinol., 14:1272–1282 (2000); and Lu. Z. L., and Hulme, E. C., J. Biol. Chem., 274:7309–7315 (1999). Additional information relative to the functional relevance of several conserved residues within GPCRs may be found by reference to Okada et al in Trends Biochem. Sci., 25:318–324 (2001).
GPCRs include receptors for sensory signal mediators (e. g., light and olfactory stimulatory molecules); adenosine, bombesin, bradykinin, endothelin, y-aminobutyric acid (GABA), hepatocyte growth factor, melanocortins, neuropeptide Y, opioid peptides, opsins, somatostatin, tachykinins, vasoactive intestinal polypeptide family, and vasopressin; biogenic amines (e. g., dopamine, epinephrine and norepinephrine, histamine, glutamate (metabotropic effect), acetylcholine (muscarinic effect), and serotonin); chemokines; lipid mediators of inflammation (e. g., prostaglandins and prostanoids, platelet activating factor, and leukotrienes); and peptide hormones (e. g., calcitonin, C5a anaphylatoxin, follicle stimulating hormone (FSH), gonadotropic-releasing hormone (GnRH), neurokinin, and thyrotropin releasing hormone (TRH), and oxytocin). GPCRs which act as receptors for stimuli that have yet to be identified are known as orphan receptors.
GPCRs are implicated in inflammation and the immune response, and include the EGF module containing, mucin-like hormone receptor (Emrl) and CD97p receptor proteins. These receptors contain between three and seven potential calcium-binding EGF-like motifs (Baud, V. et al. (1995) Genomics 26: 334–344; Gray, J. X. et al. (1996) J. Immunol. 157: 5438–5447). These GPCRs are members of the recently characterized EGF-TM7 receptors family. In addition, post-translational modification of aspartic acid or asparagine to form erythro-p-hydroxyaspartic acid or erythro-p-hydroxyasparagine has been identified in a number of proteins with domains homologous to EGF. The consensus pattern is located in the N-terminus of the EGF-like domain. Examples of such proteins are blood coagulation factors VII, IX, and X; proteins C, S, and Z; the LDL receptor; and thrombomodulin.
GPCR mutations, which may cause loss of function or constitutive activation, have been associated with numerous human diseases (Coughlin, supra). For instance, retinitis pigmentosa may arise from mutations in the rhodopsin gene. Rhodopsin is the retinal photoreceptor which is located within the discs of the eye rod cell. Parma, J. et al. (1993, Nature 365: 649–651) reported that somatic activating mutations in the thyrotropin receptor cause hyperfunctioning thyroid adenomas and suggested that certain GPCRs susceptible to constitutive activation may behave as protooncogenes.
One large subfamily of GPCRs are the olfactory receptors. These receptors share the seven hydrophobic transmembrane domains of other GPCRs and function by registering G protein-mediated transduction of odorant signals. Numerous distinct olfactory receptors are required to distinguish different odors. Each olfactory sensory neuron expresses only one type of olfactory receptor, and distinct spatial zones of neurons expressing distinct receptors are found in nasal pasages. One olfactory receptor, the RAlc receptor which was isolated from a rat brain library, has been shown to be limited in expression to very distinct regions of the brain and a defined zone of the olfactory epithelium (Raming, K. et al., (1998) Receptors Channels 6: 141–151). In another example, three rat genes encoding olfactory-like receptors having typical GPCR characteristics showed expression patterns exclusively in taste, olfactory, and male reproductive tissue (Thomas, M. B. et al. (1996) Gene 178: 1–5).
Although olfactory receptors are typically associated with olfactory function and tend to localize to the olfactory bulb, there is increasing evidence that olfactory receptors and olfactory-like receptors may play more diverse roles in varying tissues (Yuan, T, T., Toy, P., McClary, J. A., Lin, R, J., Miyamoto, N, G., Kretschmer, P, J. Gene., 278(1–2):41–51, (2001); Blache, P., Gros, L., Salazar, G., Bataille, D, Biochem, Biophys, Res, Commun., 242(3):669–72, (1998); and Matsuoka, I., Mori, T., Aoki, J., Sato, T., Kurihara, K, Biochem, Biophys, Res, Commun., 194(1):504–11, (1993)).
HM74 is a G-protein coupled receptor that was originally isolated from a human monocyte cDNA library using degenerate oligonucleotide primers devised from conserved sequences among the cDNAs encoding the human receptors for IL-8, FP and C5a (Int. Immunol. 5 (10), 1239–1249 (1993)). Although several other receptors were also isolated that appeared to represent IL-8 related cytokines and chemokines, including HM63, HM89, and HM145, HM74 appeared to share the least homology to IL-8 and was thought to be a receptor for a different type of ligand.
HM74 is also referred to as EX20 (International Publication No. WO 02/13845, filed Aug. 16, 2001). EX20 was identified as an inflammation associated protein based upon its differential expression in various leukocyte subsets upon stimulation of cytokines which are critical for regulating inflammatory processes by GM-CSF. Specifically, EX20 expression was most pronounced in macrophages and neutrophils in respiratory inflammations in COPD, emphysema, ARDS and asthma, in synovial histiomonocytes in rheumatoid arthritis, and in neutrophils and epithelioid histiocytes in Crohn's disease. A subset of lyphocytes also showed intensive signals in the airways in asthma and COPD As a result, EX20 was attributed a critical role for regulating inflammatory processes in a number of inflammatory diseases.
A splice variant of HM74 was identified and referred to as HM74A (International Publication No. WO 98/56820, filed Jun. 12, 1998).
Recently, the endogenous ligand for HM74A and HM74 was identified as nicotinic acid (International Publication No. WO 02/84298, filed Apr. 10, 2002). Nicotinic acid was identified as a ligand for both HM74A and HM74 based upon the observation that cells transfected to express HM74A and/or HM74 gain the ability to elicit Gi G protein mediated responses following exposure to nicotinic acid.
In adipose tissue a reduction of cAMP levels results in an inhibition of hormone-sensitive lipase (HSL) activity, an enzyme which regulates the process of lipolysis (i.e. the hydrolysis of triglycerides (TG) to glycerol and non-esterified fatty acids (NEFA)). Inhibition of adipocyte lipolysis resulting in a reduction of plasma NEFA levels is thought to reduce hepatic triglyceride synthesis resulting in a decreased output of TG-rich lipoproteins (VLDL). A reduction in VLDL synthesis would then produce a functional inhibition of cholesterol ester transfer protein (CETP) activity resulting in an elevation in high-density lipoprotein (HDL) levels. This alteration in lipoprotein levels, decreased TG and increased HDL, would be suitable for the treatment of dyslipidemic patients and should result in a decreased risk of cardiovascular disease.
Furthermore, inhibition of adipocyte lipolysis via the activation of a Gi-coupled receptor is thought to be an important mechanism of action of nicotinic acid (Niacin). HM74 and HM74A are believed to be such a receptor. Nicotinic acid has been used clinically for over 40 years in patients with various forms of hyperlipoproteinaemia. Nicotinic acid produces a very desirable alteration in lipoprotein profiles; reducing levels of VLDL, LDL and Lp(a) while increasing HDL. Nicotinic acid has also been demonstrated to have disease modifying benefits, reducing the progression and increasing the regression of atherosclerotic lesions and reducing the number of cardiovascular events in several trials.
HM74 and HM74A was determined to be expressed in adipose, omentum and spleen. Modulators of both receptors are believed to be useful for treating primarily cardiovascular and metabolic disorders based upon the adipose expression, in conjunction with the fact that nicotinic acid is the ligand for both receptors.
Using the above examples, it is clear the availability of a novel cloned G-protein coupled receptor provides an opportunity for adjunct or replacement therapy, and are useful for the identification of G-protein coupled receptor agonists, or stimulators (which might stimulate and/or bias GPCR action), as well as, in the identification of G-protein coupled receptor inhibitors. All of which might be therapeutically useful under different circumstances.
The present invention also relates to recombinant vectors, which include the isolated nucleic acid molecules of the present invention, and to host cells containing the recombinant vectors, as well as to methods of making such vectors and host cells, in addition to their use in the production of HGPRBMY74 polypeptides or peptides using recombinant techniques. Synthetic methods for producing the polypeptides and polynucleotides of the present invention are provided. Also provided are diagnostic methods for detecting diseases, disorders, and/or conditions related to the HGPRBMY74 polypeptides and polynucleotides, and therapeutic methods for treating such diseases, disorders, and/or conditions. The invention further relates to screening methods for identifying binding partners of the polypeptides.